1. Field of the Invention
The present invention relates to selective media for the removal of radium from aqueous sources, and in particular, to a cation resin which contains dispersed manganese oxide (DMO) and a method of manufacturing the same.
2. Brief Description of the Related Art
Radium is a divalent cation that occurs naturally in certain groundwaters, particularly in the U.S. Midwest. Radium is thought to be a carcinogen in drinking water, and, for this reason, radium levels in drinking water are regulated. For example, the U.S. Environmental Protection Agency sets a maximum contaminant level (MCL) for the total of radium 226 (Ra226) and radium 228 (Ra228) of 5.0 picocuries per liter (pCi/L).
Radium occurs at very dilute concentrations in drinking water which presents a challenge to remove radium to the extremely low regulated levels. Various processes are known, however, that are able to achieve acceptable removal efficiencies. Certain processes use the known affinity of radium for preformed hydrous manganese oxide (HMO). These conventional method use a pre-formed slurry of HMO as a coagulant which is removed with a sand filter after contacting the source water. The sludge removed in the sand filter is typically backwashed to the sewer system. Other accepted technologies for radium removal include reverse osmosis, water softener (with the brine regenerant sent to the sewer), barium doped cation resin, and the HMO-coagulation/filtration (C/F) process.
The U.S. Environmental Protection Agency reports that the Best Available Technology (BAT) for radium removal in drinking water is (a) cation exchange softening, (b) lime softening, (c) membrane processes (reverse osmosis) and (d) barium dispersed strong acid cation [1, 2]. Cation exchange softening can achieve 65-95% removal efficiency, but produces a liquid waste. Lime softening requires coagulation/filtration technology which is not considered BAT for small water supply systems, i.e., fewer than 500 connections. Lime softening can achieve 80-95% removal efficiency, but chemical precipitation produces waste solids. Membrane processes can achieve 90-99% removal efficiency, but reject water must be disposed of. Barium dispersed strong acid cation technology, such as Dowex RSC (BaSO4) [3], can achieve greater than 97% removal efficiency, but suffers from barium leakage.
Dr. Dennis Clifford of the University of Houston (a recognized expert in ground water contaminant removal) reports [3] that the BAT for radium removal is (a) cation exchange softening, (b) lime softening, (c) reverse osmosis, (d) pre-formed hydrous manganese oxide (HMO) and sand filter, (e) precipitation with BaSO4 and (f) HMO impregnated onto fibers or diatomaceous earth.
Other sorption technologies that have been tested but are not on the BAT list are:
(a) “Ion Exchange Technology—Advances In Pollution Control”, by Arup K. Sengupta,—1995 [4],
(b) weak acid cation (WAC) in the hydrogen form,
(c) activated alumina,
(d) MnO2 coated sand and
(e) naturally occurring zeolite minerals (Water Remediation Technologies [5, 7].
Calgon Carbon has developed an MnO2 impregnated media for iron and manganese removal that requires backwashing to the sewer. CalMedia™ GSR Plus [6] is a granular manganese dioxide filtering media used for reducing soluble iron and manganese. Its active surface coating oxidizes and precipitates soluble iron and manganese. A Sandia Laboratories report [6] at pp. 31-36 describes CalMedia™ GSR as a filtration media that uses the conventional HMO C/F technology.
It is known to prepare an ion exchange resin by precipitating metal oxides within the resin. For example, SenGupta et al. disclose in U.S. Pat. No. 7,291,578 a polymeric anion exchange resin in which hydrated Fe (III) oxides are dispersed within the resin beads. In one embodiment, the process involves loading the anion exchange resin with an oxidizing anion such as MnO4− or OCl− followed by passage of a ferrous sulfate solution through the resin.
The limitations of the prior art are overcome by the present invention as described below.